Optical spectrometers are utilized to quantitatively measure the concentration of a variety of analytes. Optical spectrometers utilize that portion of the spectrum commonly known as ultraviolet, visible, and infrared radiation. There are two types of commonly used spectrometers: reflectance spectrometers and transmittance spectrometers.
Reflectance spectrometers measure the concentration of an analyte when a sample absorbs and scatters portions of a sample beam. The portion of a sample beam that is not absorbed or transmitted is reflected from the sample into a detector and measured. The difference between the reflected sample beam and a reference beam quantitatively indicates the concentration of the analyte in the sample.
Similarly, transmittance spectrometers measure the concentration of an analyte when a sample absorbs a portion of a sample beam. However, the sample does not reflect the sample beam into a detector. Rather, a portion of the sample beam is absorbed as it travels through the sample. The difference between the transmitted sample beam and a reference beam quantitatively indicates the concentration of the analyte in the sample.
Spectrometers typically employ a radiation source that produces radiation with a wide frequency distribution. Tungsten filament lamps or deuterium lamps are commonly used radiation sources that produce ultraviolet (UV), visible and some infrared radiation (IR). A desired wavelength is selected through the use of filters, diffraction gratings, prisms, acousto-optic tunable filters and other means.
An acousto-optic tunable filter (AOTF) diffracts light through a sound-light interaction. This phenomenon is described by Professor Chieu D. Tran in Anal. Chem., 64:20, 971-981 (1992), incorporated herein by reference. Briefly, an AOTF is a transparent medium (e.g. quartz, tellurium dioxide (TeO.sub.2)) in which an acoustic wave can be propagated. The index of refraction of the transparent medium is perturbed by the acoustic wave as it propagates through the medium. The perturbation in the index of refraction arises from compression and rarefaction of the transparent material caused by the traveling acoustic wave. As an incident light beam passes through the transparent medium, the propagating acoustic wave produces a moving grating that diffracts portions of the incident light beam. An AOTF can be constructed such that only the first-order Bragg diffraction is observed. When only the first-order Bragg diffraction pattern is observed, two first-order beams with relative orthogonal polarizations are produced. Typically, the acoustic wave is transduced in the transparent medium by applying a radio frequency (RF) signal in the megahertz region to a piezoelectric transducer attached to the crystalline transparent medium. The use of an acousto-optic tunable filter (AOTF) to diffract a wide frequency light source to a desired frequency is advantageous in that the desired frequency can be obtained almost instantaneously, on the order of microseconds, by tuning the AOTF with a proper RF signal.
Spectrometers which utilize an acousto-optic tunable filter are known. U.S. Pat. No. 5,039,855 to Kemeny et at. describes a dual beam transmittance spectrometer which utilizes an acousto-optic tunable filter. Kemeny isolates two radiation beams from an AOTF and utilizes one beam as a reference beam and a second beam to analyze a sample (sample beam). Kemeny employs one detector to measure the reference beam and a second detector to measure the sample beam. To obtain accurate readings in spectrometers that use two detectors, the detectors must be matched because the difference between the sample beam and the reference beam measures the analyte of interest. There is a need for a dual beam transmittance spectrometer with one detector capable of measuring both a reference beam and a sample beam.
Reflectance spectrometers are known and are available commercially. Typically, in a reflectance spectrometer, a sample is illuminated at one angle relative to the sample and the reflectance is detected at a second angle relative to the sample. This configuration is preferably designed to reject specular and surface reflections from the sample so as to minimize the amount of noise reaching the detector and enhance the sensitivity of the spectrometer. Reflectance spectrometers available today are not capable of illuminating a sample at a ninety degree angle relative to the sample and detecting the reflectance from the sample at the same ninety degree angle relative to the sample. Thus, there is a need for a reflectance spectrometer capable of illuminating the sample and detecting the reflectance from the sample at the same angle (ninety degrees) relative to the sample and, at the same time, capable of minimizing the amount of specular and surface reflections reaching the detector. In addition, there is a need for a reflectance spectrometer that utilizes one or both of the orthogonally-polarized first-order beams from an AOTF.